Generally, bumpers for a vehicle are installed at the front and back of a vehicle body to protect the vehicle body and passengers as well as a hood, an engine and various devices installed in an engine room, in the event of various collision accidents including a rear-end collision.
Further, the bumper should satisfy impact requirements of every country's rule, should have high rigidity and excellent impact resistance over a wide temperature range, and should be small in expansion and contraction for a temperature change.
Such a general bumper is composed of a bumper cover defining an appearance of the bumper, a shock absorbing member maintaining a shape of the bumper cover, cushioning impact in the event of a collision and having a shape restoring function, and a back beam coming into contact with the shock absorbing member and absorbing collision energy through elastic deformation or plastic deformation.
The above-mentioned back beam is problematic in that it is formed into an arcuate shape, so that stress concentrates on a central portion in the event of a collision. Thus, the central portion of the back beam is prone to be damaged in the event of the collision.
Further, if such a back beam is formed of plastics, it is vulnerable to damage. Meanwhile, if the back beam is formed of metal, it is vulnerable to plastic deformation.